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Your LimeSurvey version: [LimeSurvey Community Edition Wersja 6.3.4+231108 ]
Own server or LimeSurvey hosting: hosting
Survey theme/template: Fruity Theme, extends_fruity
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In the previous version, trying to copy a survey resulted in an error:
500: internal server error XMLWriter::writeElement(): Argument #2 ($content) must be of type ?string, stdClass given (screenshot ls1).
Unfortunately, the last update did not fix the error.
In the list of plugins I noticed a problem with the plugin: authdb (screenshot: ls2, ls3), but I did not find any information on how to fix or reinstall it.

Does this error happen with any survey or with specific surveys?

What happens when you create a very simple small survey and try to create a copy?

The error appears when copying any survey, including newly created ones

Auth DB can not be deactivated
Seems there are PHP error in this file, then seems someone update it ?
Maybe need to ask to someone to fix it ?

Seems in 6.X : you need to go to DB to reset error